<!doctype html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta name="viewport"
          content="width=device-width, user-scalable=no, initial-scale=1.0, maximum-scale=1.0, minimum-scale=1.0">
    <meta http-equiv="X-UA-Compatible" content="ie=edge">
    <title>Document</title>
    <style>
        .header {
            width: 200px;
            height: 200px;
            margin-left: auto;
            margin-right: auto;
            background-color: skyblue;
            text-align: center;
        }
    </style>
</head>
<body>
<div class="header">
<span>里面的文字</span>
</div>
</body>
</html>